An electrical device, such as a medical device requiring the use of electricity, must comply with certain electrical safety requirements in order to achieve listing with organizations that review the safety of electrical products. A concern present for any medical device is to minimize the possibility of electrical shock caused by exposure to electrical contacts used as part of the medical device. This concern has caused the art to develop electrical circuitry such as isolation amplifiers and the like to minimize the exposure of the patient and health care practitioner to electrical voltage and current. See, for example, U.S. Pat. No. 4,803,996 (Peel et al.).
Other electrical circuitry has been developed in the circumstance when the medical device comprises two or more separable modules which require electrical contact when the modules are mated for battery-powered use. See, for example, U.S. Pat. No. 4,578,628 (Siwiak), which uses switching and fusing means to assure proper interconnection between a battery pack and its mateable radio portion and avoidance of inadvertent short circuits of the battery terminals.
Medical device market suppliers and users, in conjunction with Underwriters Laboratories, have developed safe operating performance criteria for medical apparatus that limit exposed electrical contact of apparatus to an extremely small electrical current should a person come in contact with these exposed contacts. Currently, the Underwriters Laboratories Standard for Safety 544 sets a limit of 50 microamps for double insulation circuits and otherwise 100 microamps.
Electronic medical devices often must be made small and portable. Examples include stethoscopes and other diagnostic systems, electronic prosthetic devices such as transcutaneous electrical nerve stimulators, and patient monitoring equipment. While this list is not exclusive, it generally indicates the problems which are associated with designing and producing small, portable electronic medical devices.
Requirements for such electronic medical devices can include a light-weight, portable diagnostic device having a self-contained power source (often a rechargeable battery) capable of easy and safe operation. Stationary accessory operations for such electronic medical devices typically include battery recharging apparatus and circuitry and data communication to associated computers, instruments and equipment. The mechanical alignment and electrical interconnection between the portable device and the stationary device must be convenient and safe in the critical hospital and clinical environments. These requirements are especially true of electronic stethoscopes and portable, hand held diagnostic tools such as cardiac and pulmonary auscultation detectors and graphic display apparatus.
Two examples of portable electronic medical devices having multiple modules comprising a portable diagnostic module and a stationary supporting module include stethoscopes and associated equipment disclosed in U.S. Pat. No. 5,010,889 (Bredesen et al.) and pending U.S. patent application Ser. No. 07/782,079 (Bredesen et al.), the disclosures of which are incorporated by reference.
Exposed electrical contacts are often required for making effective connections between portable devices and associated support equipment, such as a portable medical device containing a battery and the associated battery charger. Electrical safety can be achieved in various ways. Some prior efforts have included limiting the level of operating voltage, (e.g., operating below a certain threshold such as 40 volts) under carefully specified conditions as covered by Standard 544 from Underwriters Laboratories, Inc., Northbrook Ill. or providing insulation or clearances that limit leakage current to low levels and limit exposed contact points to voltages below specified levels.